Free! - Eikan wa Kimi ni Kagayaku (Doujinshi)

Matsuoka Rin is a retired Olympian who failed to win the medal he so longed for and has retreated in disgrace to become a coach at his hometown swim club. There, he meets the club's ace, 12-year-old Nanase Haruka, and decides to help Haruka realize his potential and do what Rin never could.